# The Science “Gravity” Got Right

There’s lots of bad science in the move “Gravity,” but there’s one thing they got right: microgravity. Give me a few paragraphs and this will be about weather–honest.

Most people think astronauts float in space because they’re far from the Earth’s gravitation pull. Nope. Totally wrong.

The math behind gravity bases its calculations on the distance to the Earth’s center, not its surface. Adding a few hundred miles from the center of the Earth to the astronauts is a very small change. The Hubble Space Telescope’s orbit would still leave you with 90% of the gravitational pull you get on solid ground.

That’s not enough to make you float, though you’d weigh 10% less. Where can I sign up for that?

Astronauts are weightless for the same reason you’re weightless going down that first hill on a roller coaster. Orbiting Earth actually means falling around the Earth!

By flying the Hubble or International Space Station at just the right speed their fall matches the curve of the Earth. It’s as if that first roller coaster hill never ended. And, it’s why the space shuttle always flew at around 17,000 miles per hour (give or take a little depending on their exact orbit).

In the movie, George Clooney tells Sandra Bullock space debris will reappear in around 90 minutes. That’s the time it takes to orbit the Earth a few hundred miles up at 17,000 mph. Score one for the movie.

With me so far? Good, because we’re going to 1609 for a second and Johannes Kepler. He was a German astronomer studying Mars. Don’t ask why.

Kepler came up with mathematical laws to describe all this stuff. He figured out how how fast you’d have to go at any altitude to maintain an orbit. Kepler’s laws describe the planets orbits around the Sun and today’s satellites.

Here comes the weather part.

If it takes 90 minutes to orbit the Earth at Hubble altitude, what if you moved a satellite farther away? Could you orbit the Earth every 24 hours?

24 hours is the time it takes the Earth to turn once on its axis. The orbit of the satellite and the rotation of the Earth would be the same. Because they’d be moving in sync, the satellite would seem to hover over one spot.

Using Kepler’s 500 year old laws you can calculate a satellite 22,236 miles over the equator traveling 6,878 mph will orbit Earth exactly once a day. From the ground it would look motionless.

Johannes Kepler figured out where we’d put our weather satellites 500 years ago!

By placing them in geosynchronous orbit we get 24 hour coverage to track weather systems. The satellite is always looking down on exactly the same spot on Earth.

You don’t have to think about the math. All you have to know is it works! And because it does we have a view of weather Kepler could never have dreamed of, but that you see every day on TV.

## 2 thoughts on “The Science “Gravity” Got Right”

1. Marie says:

Geoff,
It’s so confusing to me I’m lost.

2. SusyS says:

You bet! We teach all of that (ALL) to 8th graders in CT. In one day. It blows their minds at first, but they love it.